High voltage and high energy density, rechargeable (or secondary) lithium batteries based on non-aqueous electrolytes are widely used in portable devices such as camcorders, notebook computers, and cell phones. This type of battery employs cathodes comprised of lithiated transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, and variations of the previous oxides together with dopants; composition and stoichiometry of the cathode materials may vary from the foregoing. Lithium metal, lithium alloys, and carbonaceous materials are candidates as anode materials for these rechargeable lithium batteries. Carbonaceous materials are often preferred over the lithium metal and lithium alloys as anode materials in the commercial rechargeable lithium batteries. This type of lithium battery is called a lithium-ion (Li-ion) battery because no metallic lithium is present in the anode; Li ions are intercalated into and de-intercalated out of carbon materials during charge and discharge processes, respectively. The advantages of such carbon anodes are that they do not have the problems of dendrite growth on the lithium metal which can cause shorting of the cell.
Among the numerous forms of carbon materials useful in lithium-ion anodes, the most energetically favorable are the ones with a crystalline graphene structure, where the lithium ion intercalations can occur at a potential platform close to that of lithium metal, so that the corresponding cell will deliver energy at high and steady cell voltage. For this reason, almost all lithium-ion cell manufacturers/developers focus their efforts on anode materials based on graphite.
Lithium batteries, which term includes lithium ion batteries as well, require an electrolyte to operate, and this electrolyte typically comprises a solvent having an electrolyte salt dissolved therein. The graphite structure is very sensitive to the reduction and co-intercalation of most electrolyte solvents, which can cause disintegration called “exfoliation” which terminates cell operation. Therefore, the selection of solvents depends greatly on the type of carbonaceous materials used in the anode for Li-ion cells. When amorphous or graphitization retardant carbon material is used as anode material, a cyclic ester such as propylene carbonate (PC) is preferably used as the main solvent because this type of carbon is relatively non-sensitive to exfoliation, and PC has such advantages as better oxidative stability, thermal stability, and lower cost, lower melting point over other cyclic esters such as ethylene carbonate (EC). The penalty for this kind of cell chemistry is that amorphous or graphitization retardant carbon anode materials provide a lower energy density for the cells. Therefore, graphitic anode materials are more preferred.
However, it has been found that PC is not a preferred solvent in Li-ion cells when graphites or the kinds of carbons where a crystal structure (graphitization) has been developed are used as anode material. This is because PC molecules co-intercalate along with lithium-ion into the graphene structure and cause severe exfoliation, which subsequently destroys such carbonaceous anodes and yields some gaseous products inside the batteries. These problems not only shorten the life and performance of the batteries, but also raise safety concerns with such batteries because of a build-up of the internal pressure. The above problem can be resolved by using EC as the main solvent in the electrolyte. However, EC has a high melting point of 38° C. and tends to freeze out of the electrolyte at low temperatures.
Therefore, the incompatibility between PC and graphite anodes forces people to choose between a cell of higher energy density and a cell for low temperature operation. To address this problem, different approaches were adopted by researchers to increase the compatibility between PC and graphite anodes, among which the most feasible and popular is the modification of electrolyte solution by using small amount of active agents, or known as additives.
For example, Naruse et al. in U.S. Pat. No. 5,714,281 disclosed an improved non-aqueous electrolyte that employs vinylene carbonate (VC) in mixture with e.g., PC, EC, γ-butyrolactone and other linear chain esters for Li-ion cells having graphite as negative electrode (or anode). Also Simon et al. disclosed in U.S. Pat. No. 5,626,981 that using small amount of VC as an electrolyte additive can effectively suppress PC decomposition and improve the performance of the Li-ion cells with PC-containing electrolytes. This is because VC is decomposed at slightly higher potentials than PC and the resultant solid products form a stable solid electrolyte interphase (SEI), which protects PC from the further decomposition.
Similar to Naruse's approach, many other additive species were disclosed in patents and publications, including CO2, NO2, SO2, crown ethers, chloroethylene carbonate, alkyl sulfite, etc. All the additives proposed so far are molecular compounds, and most of them are either gaseous or toxic. There has never been any ionic compound (salt) that has been found to be useful in this aspect. Moreover, none of the electrolyte solutes used so far for lithium ion batteries can make any improvement in the compatibility between PC and graphite anode.
Therefore, in order to incorporate PC and graphite anode materials into a lithium-ion cell that can perform at low temperature and with high energy density, it is necessary to find an additive that can effectively form a protective SEI on graphite and prevent it from exfoliation. Moreover, it will be more advantageous if the additive found is a salt instead of a molecular compound, because the presence of the ionic additive is safer than molecular compounds in that it has much lower vapor pressure.
According to the present invention, it has been found that graphitic anodes can be stabilized by adding small amount of lithium bis(oxalato) borate ((LiB(C2O4)2) or LiBOB) or lithium oxalyldifluoroborate (LiBF2C2O4) as an additive in the above PC-based electrolytes. In addition, this invention can be generalized to other non-aqueous electrolyte systems such as the electrolyte systems containing solvents other than mixtures of linear or cyclic esters. For example, linear or cyclic ethers or esters may also be included as electrolyte components.
It is known that when a Li-ion cell constructed with any carbonaceous anode material (or negative electrode) is initially charged, some permanent capacity loss occurs due to the anode SEI formation. This permanent capacity loss is called the first cycle irreversible capacity. The SEI formation process, however, is highly dependent on the reactivity of the electrolyte components at the cell charging potentials. The electrochemical properties of the SEI film are also dependent on the chemical composition.
In order to avoid direct deposition of Li metal, Li-ion cells are generally designed to be cathode limited. Since all of the lithium-ions, which shuttle between the anode and the cathode during charging and discharging, originally come from the limited lithiated cathode, the larger the first cycle irreversible capacity, the lower the cell capacity in subsequent cycles. Thus, it is desirable to minimize or even eliminate the first cycle irreversible capacity in Li-ion cells while at the same time maintaining the high cycling capability of such cells at the low temperatures.
When PC-based electrolytes are used with a graphite anode, the first cycle irreversible capacity is much higher than it is for electrolytes without PC. Therefore, to make PC-based electrolytes practical so that lithium-ion cells for low temperature operation can be realized, it is necessary to reduce the first cycle irreversible capacity effectively so that the lithium ion consumed in the formation of SEI is relatively small, and the film thus formed will not be of high impedance. Therefore, there is also a need for an additive whose presence can reduce the first cycle irreversible capacity, and form a conductive SEI.
According to the present invention, it has been found that the first cycle irreversible capacity is reduced by adding small amount of lithium bis(oxalato) borate or lithium oxalyldifluoroborate to the state-of-the-art non-aqueous electrolytes. This is true even for PC-based electrolytes and regardless of whether or not amorphous carbon or graphite anodes are used. Li-ion cells having graphite as a negative electrode, activated with these electrolytes exhibit lower first cycle irreversible capacities relative to the cells activated with the same PC-based electrolyte without the said additive in the present invention. As a result, the cells including lithium bis(oxalato) borate or lithium oxalyldifluoroborate additive present higher subsequent cycling capacity than those without addition of the said additives.
These and other advantages of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.